Project Summary Approximately 80% of the 7 million stroke survivors in the United States are affected by hemiparesis, or muscle weakness on one side of the body. When compared to unimpaired gait, hemiparetic walking is both asymmetric and slow. High interlimb asymmetry has been documented in ankle joint power output, and preferred walking speeds following a stroke range between <0.2 m/s and ~0.8 m/s compared to ~1.4 m/s in healthy adults. Alterations in gait characteristics following a stroke are associated with limitations in walking performance including altered loading on lower limb joints and increased metabolic cost. Changes in joint loading are associated with secondary diseases including osteoarthritis, lower-back pain, and due to reduced activity, cardiovascular disease. Additionally, increases in metabolic cost lead to rapid exhaustion, less activity and limited mobility. Interventions designed to reduce mechanical asymmetries may reduce metabolic cost and normalize joint loading for improved walking performance post stroke. Ankle exoskeletons have successfully reduced metabolic cost of walking in healthy controls and preliminary studies have demonstrated the potential of ankle exoskeletons to restore ankle function by applying ankle torque on the paretic limb. The long term goal of this research is to develop a robotic ankle exoskeleton that can improve walking performance post stroke to pave the way for a portable permanent walking aid. However, exoskeleton design for stroke is limited by a knowledge gap regarding the influence of exoskeleton assistance timing and magnitude on mechanical gait symmetry, metabolic cost, and joint contact loading. Understanding this relationship is critical because exoskeleton assistance provided at the wrong time, or of insufficient magnitude could be useless, or even detrimental to gait performance. We will use a combined experimental and computational approach to research the following aims: (1) Determine how the timing and magnitude of exoskeleton assistance influence the mechanical symmetry and metabolic energetics of post-stroke walking (2) Determine how timing and magnitude of exoskeleton assistance influences joint contact forces. We will use an exoskeleton emulator system to systematically vary exoskeleton assistance parameters (i.e. timing and magnitude) while directly evaluating user metabolic, kinematic, and kinetic response. Because we cannot easily measure joint contact forces in vivo, we will apply a computational simulation approach driven by the experimentally measured gait for each participant walking with exoskeleton assistance. This work will elucidate the how timing and magnitude of exoskeleton assistance impacts post-stroke walking asymmetry, metabolic cost and joint contact forces. Taken together, these aims provide new and essential information to enable the development of exoskeletons capable of minimizing comorbidities and restoring mobility to stroke survivors.